Ultrasonic diagnostic equipment and ultrasonic image generation method

ABSTRACT

The difference frequency component between a first fundamental wave of frequency  f  and a second fundamental wave of frequency f 2  is caused to interact with a second harmonic wave, thereby to attain the enhancement of a second harmonic signal, etc., whereby a reflected wave component to be imaged is extracted at a high S/N ratio. By way of example, in a case where the difference frequency component is to appear on the lower frequency side of the second harmonic wave of the first fundamental wave so as to be superposed on this second harmonic wave, the frequencies are set at f 2 =2.8 f or so. Besides, in a case where the difference frequency component is to appear on the higher frequency side of the second harmonic wave of the first fundamental wave so as to be superposed on this second harmonic wave, the frequencies are set at f 2 =3.2 f or so.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromprior Japanese Patent Applications No. 2003-070935, filed Mar. 14, 2003;and No. 2004-067850, filed Mar. 10, 2004, the entire contents of both ofwhich are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an ultrasonic diagnostic equipment foruse in the fields of medical care, etc., and more particularly to videotechnology utilizing a nonlinear phenomenon.

2. Description of the Related Art

An ultrasonic diagnostic equipment is a medical image equipment withwhich the tomographic image of a soft tissue in a living body isnoninvasively obtained from the surface of the body by the ultrasonicpulse echo method. As compared with other medical image equipment, theultrasonic diagnostic equipment has such merits as being small-sized andinexpensive, affording a high safety without exposure to X-rays etc.,and being capable of blood flow imaging, and it is extensively utilizedfor the heart, the abdomen and the urinary organs and in obstetrics andgynecology, etc.

In the ultrasonic image diagnostic equipment, bio-information can beimaged by various imaging methods. The contrast echo method, forexample, gives an ultrasonic contrast medium made of microbubbles or thelikes, into the blood vessel of a patient, thereby to attain theenhancement of an ultrasonic scattering echo. In such an imaging method,it has hitherto been an important problem to extract an echo signalcomponent to-be-imaged at a high S/N ratio from a received signal.Therefore, various contrivances have been made in the respective imagingmethods.

Concretely, in second harmonic imaging, the band of a second harmonicwave to-be-imaged is determined by that of a fundamental wave.Accordingly, the bandwidth of the second harmonic wave is controlled byadjusting the fundamental wave band, so as to obtain a signal suitablefor the imaging. By way of example, in order to broaden the secondharmonic wave onto a lower frequency side, the fundamental wave may, inprinciple, be broadened onto the lower frequency side. Besides, in caseof broadening the second harmonic wave onto a higher frequency side, thefundamental wave may be broadened onto the higher frequency side.

In actuality, however, the band of the fundamental wave as can bebroadened onto the lower frequency side is limited by a probe band asshown in FIG. 1A. Besides, in a case where the second harmonic wave hasbeen broadened onto the higher frequency band, the bands of thefundamental wave and the second harmonic wave overlap each other in thereceived signal as shown in FIG. 1A, so that separation into therespective bands is sometimes difficult.

Besides, in the contrast echo, when a band intermediate between thefundamental wave and the second harmonic wave is imaged, the contrastbetween the tissue and the contrast medium can be heightened.

However, it is difficult to completely remove the fundamental wave andthe second harmonic wave at all times, and in the case where theintermediate band has been imaged, it is sometimes impossible to offerappropriate bio-information. By way of example, in a case where thevicinity of a 1.5 f₀ band is imaged as shown in FIG. 1B, it can occurthat parts of both the waves enter the band to-be-imaged, and that atissue exhibiting a high echo is imaged together in the contrastingmode, to make the judgment of contrast difficult.

BRIEF SUMMARY OF THE INVENTION

The present invention has been made in view of the above circumstances,and has for its object to provide an ultrasonic diagnostic equipment andan ultrasonic image generation method in which a difference frequencycomponent and a second harmonic component are caused to interact bycontrolling the phase of the difference frequency component, etc.,whereby a reflected wave component to-be-imaged is extracted at a highaccuracy.

The present invention may provide an ultrasonic diagnostic equipmentcomprising a transmission ultrasonic wave generation unit whichgenerates a transmission ultrasonic wave that has, at least, a firstfundamental wave, and a second fundamental wave at a frequency higherthan that of the first fundamental wave, and which generates thetransmission ultrasonic wave by controlling the frequency of at leastone of the first and second fundamental waves in order that, in case oftransmitting the transmission ultrasonic wave to a patient and receivinga reflected wave therefrom, a difference frequency component between thefirst fundamental wave and the second fundamental wave as is included inthe reflected wave may interact with a second harmonic wave of the firstfundamental wave, and also by controlling a phase of at least one of thefirst and second fundamental waves in order to control the interaction;a transmission unit which transmits the transmission ultrasonic wave tothe patient; a reception unit which receives the reflected wave of thetransmission ultrasonic wave from the patient; and an image generationunit which generates an ultrasonic image on the basis of the reflectedwave.

The present invention may provide an ultrasonic diagnostic equipmentcomprising a transmission ultrasonic wave generation unit whichgenerates a transmission ultrasonic wave that has, at least, a firstfundamental wave, and a second fundamental wave at a frequency higherthan that of the first fundamental wave, and which generates thetransmission ultrasonic wave by controlling the frequency of at leastone of the first and second fundamental waves in order that, in case oftransmitting the transmission ultrasonic wave to a patient and receivinga reflected wave therefrom, a sum frequency component between the firstfundamental wave and the second fundamental wave as is included in thereflected wave may interact with at least one of a second harmonic waveof the first fundamental wave and a second harmonic wave of the secondfundamental wave, and also by controlling a phase of at least one of thefirst and second fundamental waves in order to control the interaction;a transmission unit which transmits the transmission ultrasonic wave tothe patient; a reception unit which receives the reflected wave of thetransmission ultrasonic wave from the patient; and an image generationunit which generates an ultrasonic image on the basis of the reflectedwave.

The present invention may provide an ultrasonic diagnostic equipmentcomprising a transmission ultrasonic wave generation unit whichgenerates a transmission ultrasonic wave that has, at least, a firstfundamental wave, and a second fundamental wave at a frequency higherthan that of the first fundamental wave, and which generates thetransmission ultrasonic wave by controlling a phase of at least thesecond fundamental wave in order that, in case of transmitting thetransmission ultrasonic wave to a patient and receiving a reflected wavetherefrom, a difference frequency component or a sum frequency componentbetween the first fundamental wave and the second fundamental wave as isincluded in the reflected wave may cancel leakage of at least one of thefirst and second fundamental waves; a transmission unit which transmitsthe transmission ultrasonic wave to the patient; a reception unit whichreceives the reflected wave of the transmission ultrasonic wave from thepatient; and an image generation unit which generates an ultrasonicimage on the basis of the reflected wave.

The present invention may provide an ultrasonic image generation methodcomprising generating a transmission ultrasonic wave that has, at least,a first fundamental wave, and a second fundamental wave at a frequencyhigher than that of the first fundamental wave, by controlling thefrequency of at least one of the first and second fundamental waves inorder that, in case of transmitting the transmission ultrasonic wave toa patient and receiving a reflected wave therefrom, a differencefrequency component between the first fundamental wave and the secondfundamental wave as is included in the reflected wave may interact witha second harmonic wave of the first fundamental wave, and also bycontrolling a phase of at least one of the first and second fundamentalwaves in order to control the interaction; transmitting the transmissionultrasonic wave to the patient; receiving the reflected wave of thetransmission ultrasonic wave from the patient; and generating anultrasonic image on the basis of the reflected wave.

The present invention may provide an ultrasonic image generation methodcomprising generating a transmission ultrasonic wave that has, at least,a first fundamental wave, and a second fundamental wave at a frequencyhigher than that of the first fundamental wave, by controlling thefrequency of at least one of the first and second fundamental waves inorder that, in case of transmitting the transmission ultrasonic wave toa patient and receiving a reflected wave therefrom, a sum frequencycomponent between the first fundamental wave and the second fundamentalwave as is included in the reflected wave may interact with at least oneof a second harmonic wave of the first fundamental wave and a secondharmonic wave of the second fundamental wave, and also by controlling aphase of at least one of the first and second fundamental waves in orderto control the interaction; transmitting the transmission ultrasonicwave to the patient; receiving the reflected wave of the transmissionultrasonic wave from the patient; and generating an ultrasonic image onthe basis of the reflected wave.

The present invention may provide an ultrasonic image generation methodcomprising generating a transmission ultrasonic wave that has, at least,a first fundamental wave, and a second fundamental wave at a frequencyhigher than that of the first fundamental wave, by controlling a phaseof at least the second fundamental wave in order that, in case oftransmitting the transmission ultrasonic wave to a patient and receivinga reflected wave therefrom, a difference frequency component or a sumfrequency component between the first fundamental wave and the secondfundamental wave as is included in the reflected wave may cancel leakageof at least one of the first and second fundamental waves; transmittingthe transmission ultrasonic wave to the patient; receiving the reflectedwave of the transmission ultrasonic wave from the patient; andgenerating an ultrasonic image on the basis of the reflected wave.

BRIEF DESCRIPTION OF THE VIEWS OF THE DRAWING

FIG. 1A is a diagram for explaining the prior art;

FIG. 1B is a diagram for explaining the prior art;

FIG. 2A is a diagram showing the layout of an ultrasonic diagnosticequipment 10 according to a first embodiment;

FIGS. 2B and 2C are diagrams for explaining the concept of a methodaccording to this embodiment;

FIG. 3 is a diagram showing the spectrum of a first fundamental wavewhich has a frequency peak at f, and a second fundamental wave which hasa frequency peak at f₂ (f<f₂);

FIGS. 4A, 4B and 4C are diagrams for explaining the interaction betweenthe second harmonic wave of the first fundamental wave and a differencefrequency component as based on a method according to the firstembodiment;

FIG. 5 is a diagram showing the spectrum of a transmission ultrasonicwave which has two frequency peaks f and f₂ (f<f₂);

FIGS. 6A, 6B and 6C are diagrams for explaining the interaction betweenthe second harmonic wave of the first fundamental wave and a differencefrequency component as based on the method according to the firstembodiment;

FIG. 7 is a diagram showing the spectrum of a transmission ultrasonicwave which has, for example, two frequency peaks f and 3 f;

FIGS. 8A, 8B and 8C are diagrams for explaining the interaction betweenthe second harmonic wave of the first fundamental wave and a differencefrequency component as based on the method according to the firstembodiment;

FIGS. 9A and 9B are diagrams for explaining transmission ultrasonicwaves of sine type;

FIGS. 10A and 10B are diagrams for explaining transmission ultrasonicwaves of cosine type;

FIG. 11 is a flow chart showing the processing steps of an imagingmethod utilizing a difference frequency component as is executed by theultrasonic diagnostic equipment 10;

FIGS. 12A and 12B are diagrams for explaining an ultrasonic wave whichis transmitted in the first embodiment;

FIG. 13 is a diagram showing the spectrum of a transmission ultrasonicwave which is generated by adding up a first fundamental wave having afrequency peak f₁, and a second fundamental wave having a frequency peakf₂ (f₁<f₂);

FIG. 14 is a diagram showing the spectrum of reflected wave componentswhich are obtained in a case where the ultrasonic wave constituted bythe first and second fundamental waves as shown in FIG. 13 has beentransmitted to a patient;

FIG. 15 is a diagram showing the spectrum of a first fundamental wavewhich has a frequency peak at f, and a second fundamental wave which hasa frequency peak at 3 f;

FIGS. 16A, 16B and 16C are diagrams for explaining the interactionbetween the second harmonic wave of the first fundamental wave and adifference frequency component as is based on a method according to asecond embodiment;

FIGS. 17A and 17B are diagrams for explaining an ultrasonic wave whichis transmitted in the second embodiment;

FIG. 18A is a diagram showing the spectrum of a first fundamental wavewhich has a frequency peak at f, and a second fundamental wave which hasa frequency peak at 1.5 f; FIG. 18B is a diagram showing the spectrum ofthe second harmonic waves of the first fundamental wave and secondfundamental wave shown in FIG. 18A; FIG. 18C is a diagram showing thespectrum of the difference frequency component and sum frequencycomponent between the first fundamental wave and second fundamental waveshown in FIG. 18A; and FIG. 18D is a diagram showing the spectrum ofreflected waves in which the second harmonic waves shown in FIG. 18B,and the difference frequency component and sum frequency component shownin FIG. 18C are composited;

FIG. 19A is a diagram showing the spectrum of the second harmonic wavesof the first fundamental wave and second fundamental wave shown in FIG.18A; FIG. 19B is a diagram showing the spectrum of the differencefrequency component and sum frequency component between the firstfundamental wave and second fundamental wave shown in FIG. 18A; and FIG.19C is a diagram showing the spectrum of reflected waves in which thesecond harmonic waves shown in FIG. 19A, and the difference frequencycomponent and sum frequency component shown in FIG. 19B are composited;

FIG. 20A is a graph showing the spectrum of an echo signal based on amethod according to a third embodiment; and FIG. 20B shows thecomparative example of the method in FIG. 20A;

FIG. 21 is a diagram showing noise which is ascribable to thenonlinearity of a circuit;

FIGS. 22A and 22B show parts of the spectra of reflected waves obtainedin a case where an ultrasonic wave has been transmitted at 2 rates bythe prior-art method, respectively; and FIG. 22C shows a spectrumobtained by subtraction processing; and

FIGS. 23A and 23B show parts of the spectra of reflected waves obtainedin a case where an ultrasonic wave has been transmitted at b 2 rates bya method according to an embodiment, respectively; and FIG. 23C shows aspectrum obtained by subtraction processing.

DETAILED DESCRIPTION OF THE INVENTION

Now, the first embodiment-fifth embodiment of the present invention willbe described in conjunction with the drawings. By the way, in theensuing description, identical reference numerals and signs will beassigned to constituents which have substantially the same functions andconfigurations, and they shall be repeatedly explained only in necessarycases.

First Embodiment

First, the layout of an ultrasonic diagnostic equipment 10 according tothis embodiment will be described with reference to FIG. 2A. As shown inFIG. 2A, the ultrasonic diagnostic equipment 10 includes an ultrasonicprobe 11, a pulser/amplifier unit 13, a waveform control unit 14, an A/Dconverter 15, a detection unit 16, a signal processing unit 17, a filterprocessing unit 18, a filter processing unit 18 B-mode processing unit19, a Doppler processing unit 21, a DSC 23, a display unit 25, and aninput unit 27.

The ultrasonic probe 11 has a plurality of piezoelectric transducerswhich generate an ultrasonic wave on the basis of a drive signal fromthe pulser and which convert reflected waves from a patient, intoelectric signals, matching layers which are disposed for thepiezoelectric transducers, a backing material which prevents ultrasonicwaves from propagating backwards from the piezoelectric transducers, andso forth. When the ultrasonic wave is transmitted from the ultrasonicprobe 11 to the patient, various harmonic components are generated withthe propagation of the ultrasonic wave by the nonlinearity of in vivotissues. A fundamental wave and harmonic components, which constitutethe transmission ultrasonic wave, are backscattered by the boundaries ofthe acoustic impedances of the in vivo tissues, minute scatterers, etc.,and the scattered waves are received as reflected waves (echoes) by theultrasonic probe 11.

In order to form the transmission ultrasonic wave in a transmissionmode, the pulser/amplifier unit 13 recurrently generates rate pulses ata predetermined rate frequency of fr Hz (period; 1/fr second) under acontrol based on the waveform control unit 14, and it focuses theultrasonic wave into the shape of a beam every channel and gives eachrate pulse a delay time necessary for determining a transmissiondirectivity. The pulser/amplifier unit 13 impresses drive pulses on theprobe 11 at timings based on the rate pulses.

Besides, the pulser/amplifier unit 13 amplifies an echo signal acceptedthrough the probe 11, every channel in a reception mode. Further, thepulser/amplifier unit 13 gives the amplified echo signal a delay timenecessary for determining a reception directivity and then executesaddition processing in the reception mode. Owing to the addition, areflection component which arrives in a direction corresponding to thereception directivity of the echo signal is enhanced, and the overalldirectivity (scanning line) of the ultrasonic transmission and receptionis determined by the reception directivity and the transmissiondirectivity.

By the way, in a case where a digital wave former, for example, is usedin the pulse/amplifier unit 13 or the like, so as to determine thewaveform of the transmission ultrasonic wave therewith, such processingas compositing intervenes in a series of imaging, and hence, indirectnoise (the leakage component of the fundamental wave) sometimes entersinto a measured reception signal. The ultrasonic diagnostic equipment 10is capable of removing the indirect noise. This will be explained indetail later.

The waveform control unit 14 controls the pulser/amplifier unit 13 so asto form the waveform of the transmission ultrasonic wave, which isconstituted by, for example, two fundamental waves (first fundamentalwave, second fundamental wave), on the basis of an instruction from theinput unit 27, a preset program, or the like. More specifically, thewaveform control unit 14 controls the physical conditions (such asfrequency, amplitude and phase) of the second fundamental wave so thatthe difference frequency component between the second fundamental waveand the first fundamental wave may appear in the vicinity of thefrequency of the second harmonic wave of the first fundamental wave soas to interact with the second harmonic wave (so as to enhance or weakenamplitudes each other, or to give rise to phase interference in whichthe spectra of the difference frequency component and the secondharmonic wave overlap each other).

The A/D converter 15 converts an analog signal received from thepulser/amplifier unit 13, into a digital signal.

The detection unit 16 performs quadrature phase detection in such a waythat the signal received from the A/D converter 15 is multiplied bysignals having reference frequencies of a phase shift of 90 degrees,respectively, whereby I and Q signals are obtained. The I and Q signalsbecome signals which have frequencies obtained by subtracting thereference frequencies from the received signal. Incidentally, thereference frequencies are generally set at the center frequency of aband in which an ultrasonic image is generated.

The signal processing unit 17 executes such predetermined signalprocessing as subtraction processing. More specifically, the signalprocessing unit 17 executes the subtraction processing in the contrastecho after transmitting an ultrasonic wave continuously to one scanningline and receiving a plurality of reception echoes, thereby to erase atissue image and to extract a contrast medium echo component. Besides,harmonic components except the fundamental wave are extracted using asignal obtained by the phase inversion method.

The filter processing unit 18 is a complex digital filter whichattenuates reflected wave components in a predetermined frequency bandand extracts (filters) reflected wave components in a desired frequencyband, thereby to output the reflected wave components to the B-modeprocessing unit 19 or the Doppler processing unit 21. In thisembodiment, the filter processing unit 18 executes the filtering by wayof example so that the reflected wave components in the vicinity of thefrequency band of the second harmonic wave corresponding to the firstfundamental wave may be, at least, contained.

The B-mode processing unit 19 performs logarithmic amplification, etc.for the received signal subjected to the filter processing. Theamplified signal is sent to the DSC 23, and it is color-displayed on thedisplay unit 25 as a B-mode image in which the intensity of a reflectedwave is represented by brightness.

The Doppler processing unit 21 extracts a blood flow or tissue and thecontrast medium echo component on the basis of the Doppler effect, andfinds a mean speed, variance, power, and the like blood flow informationat multiple points. The blood flow information items are sent to the DSC23, and are color-displayed on the display unit 25 as a mean speedimage, a variance image, a power image, and an image in which they arecombined.

The DSC 23 converts the scanning line signal train of ultrasonic scaninputted from the B-mode processing unit 19 or the Doppler processingunit 21, into the data of a rectangular coordinate system based onspatial information, and it further performs a video format change.

The display unit 25 displays in vivo morphological information or theblood flow information as an image on the basis of the video signal fromthe DSC 23. Besides, in the case of employing the contrast medium, thedisplay unit 25 displays a brightness image or a color image on thebasis of a quantitative information content from which the spatialdistribution of the contrast medium, that is, the blood flow or a regionwhere blood exists has been obtained.

The input unit 27 is connected to the body of the equipment 10, and itis furnished with (a mouse or track ball, a mode changeover switch, akeyboard, etc.) for the setting of a region of interest (ROI), etc. inorder to accept various instructions, commands and information from anoperator into the equipment body 22. Besides, the transmissionconditions of the transmission ultrasonic wave can also be manuallyinputted through the input unit 27.

(Difference Frequency Component)

Next, there will be described the principle of generating a differencefrequency component which is utilized in the imaging method. In general,in a case where a fundamental wave is denoted by a·sin ft, a squarenonlinear effect can be expressed as (a·sin ft)². Besides, in case ofusing transmission ultrasonic waves which consist of a first fundamentalwave of sin f₁t and a second fundamental wave of a·sin f₂t, thenonlinear effect can be expressed as (sin f₁t+a·sin f₂t)², which can bereduced as follows: $\quad\begin{matrix}{\left( {{\sin\quad f_{1}t} + {{a \cdot \sin}\quad f_{2}t}} \right)^{2} = {\left( {\sin\quad f_{\quad 1}\quad t} \right)^{2}\underset{\_}{+}{2{a \cdot \sin}\quad f_{\quad 1}{t \cdot \sin}\quad f_{\quad 2}t} +}} \\{\left( {{a \cdot \sin}\quad f_{\quad 2}\quad t} \right)^{2}} \\{= {{{1/2}\left\{ {{\sin\quad 2f_{\quad 1}t}\overset{\_}{+}{{a \cdot \sin}\left( {f_{\quad 2} - f_{\quad 1}} \right)t}} \right\}} + \cdots}}\end{matrix}$

Here, the first term of the last line is the second harmonic componentof the first fundamental wave, and the second term corresponds to thedifference frequency component between the second fundamental wave andthe first fundamental wave. Besides, fundamental wave components, sumfrequency components, etc. are included in terms omitted at the thirdline.

With the method, the interaction between the difference frequencycomponent and the second harmonic component is controlled by controllingthe frequency and phase of the fundamental wave, whereby a reflectedwave component to be imaged is extracted at a high S/N ratio.Incidentally, a method which extracts and images the differencefrequency component itself is stated in, for example, Japanese PatentApplication No. 2001-110307, but any statement pertaining to the phasecontrol is not contained in the document.

Next, there will be described an imaging method utilizing the differencefrequency component in accordance with this embodiment. First, theconcept of the imaging method will be elucidated with reference to FIG.2A-FIG. 8C.

FIG. 2B is a diagram showing the spectrum of ultrasonic waves which havetwo frequency peaks f₁ and f₂ (where f₁<f₂) (hereinbelow, the formerultrasonic wave shall be called “first fundamental wave”, and the latterultrasonic wave “second fundamental wave”). FIG. 2C is a diagram showingthe spectrum of reflected waves (a reception signal) obtained in a casewhere a transmission ultrasonic wave constituted by the firstfundamental wave and second fundamental wave has been transmitted to apatient. As shown in FIG. 2C, the reception signal contains thereflected waves which correspond to the fundamental wave components ofthe frequencies f₁, f₂ constituting the transmission ultrasonic wave,and besides, the difference frequency components (DC, f₂−f₁) and sumfrequency components (2 f₁, 2 f₂, f₁+f₂) of the reflected wavesgenerated by the nonlinearity of in vivo propagation. Here, “DC” denotesa frequency component within a band which is somewhat broad centeringround a zero frequency. Further, the sum frequency components of thefrequencies 2 f₁, 2 f₂ correspond to the second harmonic wave of thefirst fundamental wave and that of the second fundamental wave,respectively.

In the method according to this embodiment, note is taken of, forexample, the second harmonic component of the first fundamental wave andthe difference frequency component of the frequency (f₂−f₁). Morespecifically, the frequency and phase of the second fundamental wave arecontrolled so as to superpose the difference frequency component of thefrequency (f₂−f₁) on the second harmonic component of the firstfundamental wave in inphase fashion. Thus, the component in the vicinityof the second harmonic band of the first fundamental wave is enlargedand imaged, thereby to obtain useful bio-information.

In accordance with this method, the magnitude of the reflected wavecomponent in the vicinity of the second harmonic band of the firstfundamental wave can be controlled in the three aspects of a lowerfrequency side, a higher frequency side and both the sides with respectto the second harmonic band. The respective aspects will be concretelyexplained below.

FIG. 3 is a diagram showing the spectrum of a first fundamental wavewhich has a frequency peak at f, and a second fundamental wave which hasa frequency peak at f₂ (f<f₂). Herein, the frequency peak f₂ is assumedto be a value being somewhat smaller than 3 f, for example, a value ofabout 2.8 f. In a case where a transmission ultrasonic wave constitutedby the fundamental waves having the spectrum has been transmitted to apatient, a reflected wave shown in FIG. 4C, in which the second harmonicwave of the first fundamental wave as shown in FIG. 4A and a differencefrequency component (center frequency) shown in FIG. 4B are added up, ismeasured in the vicinity of a frequency 2 f. The reflected wave isequivalent to a wave in which the second harmonic component of the firstfundamental wave as has the center frequency of 2 f is broadened onto alower frequency side so as to enlarge its peak.

Besides, FIG. 5 is a diagram showing the spectrum of a transmissionultrasonic wave which has, for example, two frequency peaks f and f₂(f<f₂). Herein, the frequency peak f₂ is assumed to be a value beingsomewhat larger than 3 f, for example, a value of about 3.2 f. In a casewhere ultrasonic waves having the spectrum have been transmitted to apatient, a reflected wave shown in FIG. 6C, in which the second harmonicwave of the first fundamental wave as shown in FIG. 6A and a differencefrequency component (center frequency) shown in FIG. 6B are added up, ismeasured in the vicinity of a frequency 2 f. The reflected wave isequivalent to a wave in which the second harmonic wave of the firstfundamental wave as has the center frequency of 2 f is broadened onto ahigher frequency side so as to enlarge its peak.

Further, FIG. 7 is a diagram showing the spectrum of a transmissionultrasonic wave which has, for example, two frequency peaks f and 3 f.Herein, a fundamental wave which has its peak at 3 f is such that thefundamental wave having its peak at f₁ in FIG. 3, and the fundamentalwave having its peak at f₂ in FIG. 5 are added up. In a case whereultrasonic waves having the spectrum have been transmitted to a patient,a reflected wave shown in FIG. 8C, in which the second harmonic wave ofthe first fundamental wave as shown in FIG. 8A and a differencefrequency component (center frequency) shown in FIG. 8B are added up, ismeasured in the vicinity of a frequency 2 f. The reflected wave isequivalent to a wave in which the second harmonic wave of the firstfundamental wave as has the center frequency of 2 f is broadened onto alower frequency side and a higher frequency side so as to enlarge itspeak.

(Transmission Ultrasonic Wave)

Next, there will be explained a transmission ultrasonic wave for causingthe difference frequency component to appear. The transmissionultrasonic waves can be classified into the sine type and the cosinetype, depending upon whether they are in even symmetry or in oddsymmetry with respect to the origin which is set at the center oftransmission pulses. Besides, each of the types includes a subtype whichgenerates a difference frequency component that enhances the secondharmonic wave of a fundamental wave (that is, a subtype which generatesa difference frequency component that has the same polarity as that ofthe second harmonic wave), and a subtype which generates a differencefrequency component that weakens the second harmonic wave (that is, asubtype which generates a difference frequency component that has theopposite polarity to that of the second harmonic wave). Now, the sinetype will be explained with reference to FIGS. 9A and 9B, and the cosinetype with reference to FIGS. 10A and 10B.

FIG. 9A shows the waveform (central part) of a transmission ultrasonicwave of the sine type for generating the difference frequency componentwhich enhances the second harmonic wave. As shown in the figure, inorder to generate the difference frequency component which enhances thesecond harmonic wave, a second fundamental wave (frequency f₂) has itsphase controlled so that the crest (trough) of the amplitude of thesecond fundamental wave may, for example, agree in timing with the crest(trough) of the amplitude of a first fundamental wave (frequency f₁). Onthe other hand, FIG. 9B shows the waveform (central part) of atransmission ultrasonic wave of the sine type for generating thedifference frequency component which weakens the second harmonic wave.As shown in the figure, in order to generate the difference frequencycomponent which weakens the second harmonic wave, a second fundamentalwave (frequency f₂) has its phase controlled so that the crest (trough)of the amplitude of the second fundamental wave may, for example, agreein timing with the trough (crest) of the amplitude of a firstfundamental wave (frequency f₁).

More specifically, letting the first fundamental wave be Ψ₁=A₁sin(2πωf₁), and the second fundamental wave be Ψ₂=A₂ sin(2πωf₂+θ), acondition for enhancing or weakening the first fundamental wave and thesecond fundamental wave can be expressed in terms of an initial phase θ(or a phase at the origin defined above), as follows:

Subject to A₁, A₂>0,

-   (1) Enhancing condition θ=π-   (2) Weakening condition θ=0 or 2π

Meanwhile, FIG. 10A shows the waveform (central part) of a transmissionultrasonic wave of the cosine type for generating the differencefrequency component which enhances the second harmonic wave. As shown inthe figure, in order to generate the difference frequency componentwhich enhances the second harmonic wave, a second fundamental wave(frequency f₂) has its phase controlled so that the crest (trough) ofthe amplitude of the second fundamental wave may, for example, agree intiming with the crest (trough) of the amplitude of a first fundamentalwave (frequency f₁). On the other hand, FIG. 10B shows the waveform(central part) of a transmission ultrasonic wave of the cosine type forgenerating the difference frequency component which weakens the secondharmonic wave. As shown in the figure, in order to generate thedifference frequency component which weakens the second harmonic wave, asecond fundamental wave (frequency f₂) has its phase controlled so thatthe crest (trough) of the amplitude of the second fundamental wave may,for example, agree in timing with the trough (crest) of the amplitude ofa first fundamental wave (frequency f₁).

More specifically, letting the first fundamental wave be ψ₁=A₁cos(2πωf₁), and the second fundamental wave be ψ₂=A₂ cos(2πωf₂+θ), acondition for enhancing or weakening the first fundamental wave and thesecond fundamental wave can be expressed in terms of an initial phase θ,as follows:

Subject to A₁, A₂>0,

-   (1) Enhancing condition θ=0 or 2π-   (2) Weakening condition θ=π

Incidentally, this embodiment will refer to the case where the secondharmonic wave and the difference frequency component enhance each otheras will be stated later. Accordingly, the transmission ultrasonic wavehaving the waveform shown in FIG. 9A or FIG. 10A (more strictly,ultrasonic wave obtained by superposing the fundamental waves shown ineach figure; refer to FIG. 12) is generated by the pulser/amplifier unit13 under the control of the waveform control unit 14, and it istransmitted from the probe 11. Besides, in the second embodiment whichdescribes the case where the second harmonic wave and the differencefrequency component weaken each other as will be stated later, thetransmission ultrasonic wave having the waveform shown in FIG. 9B orFIG. 10B (refer to FIG. 17) is generated by the pulser/amplifier unit 13under the control of the waveform control unit 14, and it is transmittedfrom the probe 11.

(Imaging Method Utilizing Difference Frequency Component)

Next, there will be described the operation of the ultrasonic diagnosticequipment 10 in the case of executing an imaging method which utilizes adifference frequency component. This embodiment consists in directlycontrolling the phase of the second harmonic wave of a fundamental wave,etc., and indirectly controlling the phase of the difference frequencycomponent, thereby to enlarge a harmonic component near the secondharmonic band of the first fundamental wave. This embodiment isprofitable in case of imaging, for example, a tissue region.

FIG. 11 is a flow chart showing the processing steps of the imagingmethod utilizing the difference frequency component as is executed bythe ultrasonic diagnostic equipment 10. Referring to the figure, firstof all, the conditions of a transmission ultrasonic wave areautomatically set (step S1). Here, the “conditions of the transmissionultrasonic wave” signify the frequencies, amplitudes, phases and otherphysical conditions of first and second fundamental waves for causingthe difference frequency component between the second fundamental waveand the first fundamental wave to appear near the frequency of thesecond harmonic wave of the first fundamental wave. Incidentally, thefrequencies being the conditions of the transmission ultrasonic wave maywell be preset as the multiple frequencies of a harmonic mode. Anoperator can select any frequency at will through the input unit 27 fromamong a plurality of recommended frequencies which are displayed on thedisplay unit 25 for individual purposes.

Concretely, setting as stated below is automatically performed at thestep S1. In a case where the difference frequency component is to appearin the vicinity and on the lower frequency side of the frequency of thesecond harmonic wave of the first fundamental wave (refer to FIG. 3 andFIG. 4), the frequency of the second fundamental wave is set at, forexample, 2.8 f where f is let denote the frequency of the firstfundamental wave. On the other hand, in a case where the differencefrequency component is to appear in the vicinity and on the higherfrequency side of the frequency of the second harmonic wave of the firstfundamental wave (refer to FIG. 5 and FIG. 6), the frequency of thesecond fundamental wave is set at, for example, 3.2 f where f is letdenote the frequency of the first fundamental wave. Besides, in a casewhere the difference frequency component is to appear in the vicinityand on the lower and higher frequency sides of the frequency of thesecond harmonic wave of the first fundamental wave (refer to FIG. 7 andFIG. 8), the frequency of the second fundamental wave is set at, forexample, 3 f where f is let denote the frequency of the firstfundamental wave. Further, the polarities and amplitudes of the firstand second fundamental waves are controlled so as to enhance the secondharmonic wave and the difference frequency component each other.

FIG. 12A shows examples of the ultrasonic pulses of the respectivefundamental waves in the case of n=6 (where n denotes a wave number). Aburst wave concerning the first fundamental wave and a burst waveconcerning the second fundamental wave as shown in the figure are addedup, whereby a transmission ultrasonic wave shown in FIG. 12B isgenerated.

Incidentally, apart from the automatic setting stated above, the settingof the conditions of the fundamental waves at the step S1 may well be soschemed that the operator manually sets the conditions on the basis of,for example, patient information, diagnostic information containing animaging part, and the selection of an imaging mode.

Subsequently, a transmission ultrasonic wave shown in FIG. 12B istransmitted into a patient (step S2), and the resulting reflected waveis received as an echo signal (step S3). The echo signal has a peakconstituted by the second harmonic wave of the first fundamental waveand the difference frequency component, in the vicinity of a frequencyband of 2 f.

After undergoing processing such as amplification and delay addition,the echo signal is subjected to filtering by the filter processing unit18 (step S4). By way of example, the filtering proceeds in such a mannerthat the echo signal is passed through a band centering at a frequencyof 2 f (a band in which a tissue harmonic echo component ispredominant), and that bands before and behind the above band are allattenuated.

Subsequently, the filtered echo signal is subjected to predeterminedprocessing in the B-mode processing unit 19 (or the Doppler processingunit 21) (step S5), and the resulting signal is displayed as anultrasonic image on the display unit 25 (step S6).

In accordance with the configuration stated above, advantages to bestated below can be attained.

In accordance with the method according to this embodiment, the phase ofthe difference frequency component between the fundamental wavesconstituting the transmission ultrasonic wave, etc. are controlled,whereby the difference frequency component and the second harmoniccomponent can interact so as to enhance each other. Thus, the secondharmonic component can be enlarged on its lower frequency side or/andhigher frequency side, and the reflected wave component to-be-imaged canbe enhanced. As a result, the method can heighten the versatility ofband design as compared with the prior-art second harmonic imaging.Moreover, an imaging signal can be extracted at a high S/N ratio, anduseful bio-information can be offered at medical sites.

Besides, in accordance with the ultrasonic diagnostic equipmentaccording to this embodiment, the phase of the difference frequencycomponent between the fundamental waves constituting the transmissionultrasonic wave, etc. can be set, not only manually, but alsoautomatically with the diagnostic information etc. Moreover, since therecommended combinations of the individual fundamental wave frequenciesare displayed for the respective purposes, the setting of thefrequencies for the interaction between the difference frequencycomponent and the second harmonic component can be performed easily andquickly.

Second Embodiment

Now, the second embodiment of the present invention will be described.The second embodiment consists in controlling the phase of a differencefrequency component so as to remove or reduce the second harmoniccomponent of a first fundamental wave. This example is profitable in acase, for example, where, as disclosed in Japanese Patent ApplicationNo. 2001-343577, in the contrast echo, a tissue image and a contrastmedium echo are separated in a band which is about 1.5 times thefrequency of the first fundamental wave, so as to image the contrastmedium echo.

The general concept of an imaging method utilizing a differencefrequency component will be described with reference to FIGS. 13 and 14.

FIG. 13 is a diagram showing the spectrum of a transmission ultrasonicwave which is generated by adding up a first fundamental wave having afrequency peak f₁, and a second fundamental wave having a frequency peakf₂ (F₁<f₂). Incidentally, the first fundamental wave and secondfundamental wave have the weakening relationship shown in, for example,FIG. 9B or FIG. 10B.

Besides, FIG. 14 is a diagram showing the spectrum of reflected wavecomponents which are obtained in a case where the ultrasonic waveconstituted by the first and second fundamental waves as shown in FIG.13 has been transmitted to a patient. As shown in FIG. 14, the reflectedwave components contain reflected waves which correspond to thefundamental wave components of the frequencies f₁, f₂ constituting thetransmission ultrasonic wave, and besides, the difference frequencycomponents (DC, f₂−f₁) and sum frequency components (2 f₁, 2 f₂, f₁+f₂)of reflected waves generated by the nonlinearity of in vivo propagation.Here, the sum frequency components of the frequencies 2 f₁, 2 f₂correspond to the second harmonic wave of the first fundamental wave andthat of the second fundamental wave, respectively.

In the second embodiment, the transmission frequency f₂ is controlled soas to equalize the frequencies (f₂−f₁) and 2 f₁, whereby the secondharmonic component of the first fundamental wave is cancelled by thedifference frequency component of the frequency (f₂−f₁). Thus, in thecontrast echo by way of example, the tissue harmonic component in thevicinity of a 2 f₁ band can be removed, and the reflected wave componentfrom a contrast medium can be extracted at a high accuracy.

FIG. 15 is a diagram showing the spectrum of a first fundamental wavewhich has a frequency peak at f, and a second fundamental wave which hasa frequency peak at 3 f. In a case where a transmission ultrasonic waveconstituted by the fundamental waves having the spectrum has beentransmitted to a patient, a reflected wave shown in FIG. 16C, in whichthe second harmonic wave of the first fundamental wave as shown in FIG.16A and a difference frequency component (center frequency) shown inFIG. 16B are added up, is measured in the vicinity of a frequency 2 f.Incidentally, the second harmonic wave of the first fundamental wave isideally cancelled by the difference frequency component (becomes zero).

Next, the operation of the ultrasonic diagnostic equipment 10 in thecase of executing an imaging method which utilizes a differencefrequency component will be described with reference to FIG. 11.

First, the conditions of a transmission ultrasonic wave are set throughthe input unit 27 or automatically (step S1).

FIG. 17A shows examples of the ultrasonic pulses of the respectivefundamental waves in the case of n=6. A first burst wave concerning thefirst fundamental wave and a second burst wave concerning the secondfundamental wave (however, phases are inverted to those in the case ofFIG. 12B) as shown in the figure are added up, whereby the transmissionultrasonic wave shown in FIG. 17B is generated.

Incidentally, the setting of the conditions of the transmissionultrasonic wave at this step may be realized by the automatic setting orselection setting stated in the first embodiment.

Thenceforth, substantially the same processing as in the firstembodiment is performed, whereby a reflected wave component with asecond harmonic component cancelled by a difference frequency componentcan be extracted at a high accuracy. The extracted contrast medium echois imaged, whereby useful bio-information can be obtained quickly andeasily.

Third Embodiment

Next, the third embodiment will be described. This embodiment consistsin that the sum frequency component between a first fundamental wave anda second fundamental wave is caused to interact with the second harmonicwave of the first fundamental wave or second fundamental wave, therebyto realize imaging in a wide band, imaging at a high S/N ratio, etc.Now, the interaction in which the sum frequency component and the secondharmonic wave are superposed and intensified each other will beexplained with reference to FIGS. 18-20 by way of example.

FIG. 18A is a diagram showing the spectrum of a first fundamental wavewhich has a frequency peak at f, and a second fundamental wave which hasa frequency peak at 1.5 f. In a case where a transmission ultrasonicwave constituted by the fundamental waves having the spectrum has beentransmitted to a patient, there are generated the second harmonic wavesof the first and second fundamental waves as have a spectrum shown inFIG. 18B, and a difference frequency component and a sum frequencycomponent having a spectrum shown in FIG. 18C. Accordingly, whenreflected waves in which the second harmonic waves and the differenceand sum frequency components are composited are received, the spectrumthereof comes to have a wide band as shown in FIG. 18D by way ofexample.

Besides, as shown in FIGS. 19A and 19B, the second harmonic waves,difference frequency component and sum frequency component which areobtained by the ultrasonic transmission appear in bands wider than inFIGS. 18B and 18C, in some cases. In such a case, when the reflectedwaves in which they are composited are received, the spectrum of theharmonic components with the fundamental waves excluded comes to have awide band in which the difference frequency component is also superposedas shown in FIG. 19C by way of example.

Next, the operation of the ultrasonic diagnostic equipment 10 in thecase of executing an imaging method which utilizes the sum frequencycomponent will be described with reference to FIG. 11. This embodimentconsists in directly controlling the frequency of the first fundamentalwave and the phase and frequency of the second fundamental wave, andindirectly controlling the frequency and phase of the sum frequencycomponent, thereby to widen the band of the reflected waves. Thisembodiment is profitable in a case, for example, where harmoniccomponents derived by phase inversion are subjected to frequencycompounding and are then imaged. Here, the “frequency compounding”signifies that a received signal is divided into a plurality offrequency bands, that the signals of the respective bands are subjectedto filter processing and B-mode processing, and that the processedresults are composited (added).

Referring to FIG. 11, first of all, the conditions of a transmissionultrasonic wave are automatically set (step S1). Here, the “conditionsof the transmission ultrasonic wave” signify the frequencies,amplitudes, phases and other physical conditions of first and secondfundamental waves for causing the sum frequency component between thesecond fundamental wave and the first fundamental wave to appear in thevicinity of the higher frequency side of the second harmonic wave of thefirst fundamental wave. Incidentally, the frequencies being theconditions of the transmission ultrasonic wave are preset as themultiple frequencies of a harmonic mode, as in the case of thedifference frequency component.

Concretely, setting as stated below is automatically performed at thestep S1. In order to superpose the sum frequency component on the secondharmonic waves of the first fundamental wave and second fundamentalwave, the center frequency of the first fundamental wave is set at, forexample, f₁=1.6 MHz, and that of the second fundamental wave is set at,for example, f₂=2.5 MHz. Besides, the polarities and amplitudes of thefirst and second fundamental waves are controlled so as to enhance thesecond harmonic waves and the sum frequency component each other.

Subsequently, the transmission ultrasonic wave is transmitted into apatient (step S2), and the resulting reflected waves are received as anecho signal (step S3).

FIG. 20A is a graph showing the spectrum of the echo signal obtained insuch a way that the transmission ultrasonic wave constituted by theabove fundamental waves has its polarity inverted and is thentransmitted at 2 rates (pulse inversion transmission). As shown in thefigure, owing to the interaction between the sum frequency component andthe second harmonic waves, the spectrum C of harmonics (in which thedifference frequency component, the second harmonic wave of the firstfundamental wave, and the sum frequency component are superposed, andwhich shall be called “harmonic component” below) in the echo signal canbe caused to appear in a wide band. Accordingly, bands D and E for usein the frequency compounding can be set wide.

On the other hand, FIG. 20B is a graph showing the spectrum of an echosignal obtained in such a way that an ultrasonic wave having a centerfrequency f=2.1 MHz in the figure has its polarity inverted and is thentransmitted at 2 rates (pulse inversion transmission). In the ultrasonictransmission, a sum frequency component and second harmonic componentsdo not interact. As seen from the figure, a harmonic component C in anecho signal appears in a band width which is narrower than in the caseof FIG. 20A where the sum frequency component and the second harmonicwaves interact. Accordingly, a distance resolution is not satisfactory,and a speckle reducibility is not high, either.

After the respective bands of the obtained harmonic component asindicated at D and E in FIG. 20A have undergone such processing asamplification and delay addition, they are subjected to filtering by thefilter processing unit 18 (step S4). The filtering can be performed incomparatively wide bands as shown in FIG. 20A by way of example.

Subsequently, the individual filtered echo signals undergo predeterminedprocessing in the B-mode processing unit 19 (or the Doppler processingunit 21) (step S5), and they are thereafter composited so as to bedisplayed as an ultrasonic image on the display unit 25 (step S6).

In accordance with the method according to this embodiment, thefrequency, phase etc. of the first fundamental wave or secondfundamental wave are controlled, whereby the sum frequency component andthe second harmonic component can interact so as to enhance each other.Thus, the second harmonic wave of the first fundamental wave can beenlarged on its higher frequency side, and the second harmonic wave ofthe second fundamental wave on its lower frequency side, so that thereflected wave component to-be-imaged is enhanced. As a result, adistance resolution can be enhanced. Moreover, since the differencebetween the frequencies to be compounded is large, a highly effectivespeckle reducibility can be realized.

Incidentally, this embodiment has been described by exemplifying theinteraction in which the sum frequency component and the second harmonicwave are superposed and intensified each other. In contrast, it is alsopossible to give rise to an interaction in which the sum frequencycomponent and the second harmonic wave are cancelled each other. Forthis purpose, the phase of at least one of the first and secondfundamental waves may be controlled so as to endow the fundamental waveswith the weakening relationship as shown in FIG. 9B or FIG. 10B, by wayof example.

Fourth Embodiment

Next, the fourth embodiment of the present invention will be described.The fourth embodiment consists in that the phase control of thedifference frequency component as explained in the first embodiment, orthe phase control of the sum frequency component as explained in thethird embodiment is executed in a Doppler mode. Now, the phase controlof the difference frequency component as explained in the firstembodiment (that is, the case where the difference frequency componentand the second harmonic wave enhance each other) will be taken as anexample and described with reference to FIG. 1.

In general, in the Doppler mode, a plurality of times of ultrasonicreceptions are performed for a single scanning line. In each oftransmissions for the single scanning line, the waveform control unit 14transmits an ultrasonic wave which generates a difference frequencycomponent that enhances a second harmonic wave. As already stated, thetransmission ultrasonic wave is generated by superposing a firstfundamental wave and a second fundamental wave the crests (or troughs)of which are inphase to each other.

When a reception echo signal corresponding to each transmissionultrasonic wave has been received, the signal processing unit 17extracts the signal component of a second harmonic band (that is, asignal component in which the difference frequency component issuperposed on the second harmonic wave) from the reception echo signal.

The Doppler processing unit 21 extracts contrast medium echo componentsbased on the Doppler effect, on the basis of such extracted signalcomponents of the second harmonic bands, and it finds a mean speed,variance, power, and the like information at multiple points. Theinformation items are sent to the DSC 23, and are color-displayed on thedisplay unit 25 as a mean speed image, a variance image, a power image,and a Doppler image in which they are combined.

Thus far, there has been exemplified the case where, in the Dopplermode, the difference frequency component and the second harmonic waveinteract so as to enhance each other. In contrast, in a case where adifference frequency component and a second harmonic wave interact so asto weaken each other, an ultrasonic wave which generates the differencefrequency component that weakens the second harmonic wave is transmittedin each of a plurality of times of ultrasonic transmissions for a singlescanning line. As already stated, the transmission ultrasonic wave isset by superposing a first fundamental wave and a second fundamentalwave the crest of one of which and the trough of the other of which areinphase.

In accordance with the above configuration, even in the Doppler mode,the same advantages as in the first embodiment can be attained. Thisembodiment is especially profitable in the imaging of, for example, thecoronary arteries.

Fifth Embodiment

Next, the fifth embodiment of the present invention will be described.This embodiment consists in that the phase, amplitude and polarity of adifference frequency component or a sum frequency component arecontrolled, thereby to cancel the leakage of a fundamental wave by thedifference frequency component. The leakage of the fundamental wave isascribable to characteristics inherent in the ultrasonic diagnosticequipment 10. Now, a case of utilizing the difference frequencycomponent will be taken as an example and described.

In general, each electric or electronic circuit for use in the equipmentshould preferably be such that the input and output thereof are linear,from the viewpoints of control etc. However, each circuit does not havea linear input-output relationship in a strict sense, but it involves anonlinearity. Especially in case of using a digital circuit, thenonlinearity of the circuit is induced by compositing in the conversionof a digital signal into an analog signal, etc.

Also in the ultrasonic diagnostic equipment 10, the leakage of afundamental wave as shown in FIG. 21 is sometimes induced for the reasonthat a reception signal is influenced by the nonlinearity of, forexample, a digital wave former used in the transmission/receptioncircuit 15 or a circuit used in any other constituent.

With the method according to this embodiment, the leakage of thefundamental wave attributed to the nonlinearity of the circuit andprecision limitation is removed or reduced by the difference frequencycomponent. By way of example, in case of removing a noise componentshown in FIG. 21, an ultrasonic wave constituted by a first fundamentalwave whose frequency peak lies at f, and a second fundamental wave whosefrequency peak lies at 2.5 f, is transmitted at ultrasonic transmissionso that the difference frequency component may appear near 1.5 f. Onthis occasion, the amplitude of the second fundamental wave is so thatthe difference frequency component between the first and secondfundamental waves may become substantially equal to the noise componentascribable to the nonlinearity of the circuit, and the polarity of thesecond fundamental wave is set so as to become inverse to the polarityof the first fundamental wave (refer to the first embodiment).

Owing to such a configuration, the difference frequency component can becaused to appear as a corresponding spectrum in a band in which thenoise component ascribable to the nonlinearity of the circuit is to beremoved. In the reception signal, accordingly, the noise component andthe difference frequency component are cancelled from each other, andthe noise component can be removed or reduced.

Sixth Embodiment

Next, the sixth embodiment will be described. This embodiment consistsin applying the technological idea of the present invention to a casewhere a plurality of times of ultrasonic transmissions/receptions areperformed in the contrast echo, and where a contrast medium echo isextracted by a subtraction method for the purpose of deleting a motionartifact. Now, a case of utilizing a difference frequency component willbe taken as an example and described.

In general, a reflected wave to be received is influenced by the motionof a patient. The motion appears as a motion artifact on an image in,for example, an imaging method which employs a plurality of times ofultrasonic transmissions/receptions. In this embodiment, the phase ofthe difference frequency component, etc. are controlled, thereby toremove or reduce the leakage component of a fundamental wave as arisesin each reception ultrasonic wave, and the difference of echo signals istaken between different rates, thereby to remove or reduce the motionartifact and the like noise component which arise.

More specifically, first of all, a transmission ultrasonic wave which isgenerated by adding up a first fundamental wave having a frequency peakf, and a second fundamental wave having a frequency peak f₂ (f<f₂), istransmitted at, at least, 2 rates at predetermined timings. Thetransmission ultrasonic wave is generated so that the phases of thesecond harmonic wave and the difference frequency component may bereversed. Besides, in the ensuing description, the transmissionultrasonic wave shall be transmitted at 2 rates.

When an echo signal corresponding to each transmission ultrasonic waveis received, it is subjected to predetermined processing such asamplification and A/D conversion. To be noted here is that, in each echosignal, the leakage component of the fundamental wave has been removedor reduced by the difference frequency component.

The received echo signals undergo the predetermined processing such asamplification and A/D conversion, and they are subjected to subtractionbetween the different rates by the signal processing unit 17, whereby acontrast medium echo signal is extracted. This operation is based on aprinciple stated below.

In the reception signal obtained at each rate, components which areother than a contrast medium echo component originate from tissues. Onthe other hand, the contrast medium echo component originates from acontrast medium in the body fluid. Since the contrast medium undergoeschanges (such as rupture) by the transmission ultrasonic wave, thetemporal signal transition of the contrast medium echo component betweenthe rates is considered to be larger as compared with that of thecomponents originating from the tissues. Accordingly, when the receptionsignal of the first rate is subtracted from that of the second rate byway of example, the components originating from the tissues consist onlyof a component originating from the motion, and the contrast medium echocomponent having a higher spectrum is relatively enhanced. Effectiveblood flow information can be imaged by imaging the enhanced component.

In this embodiment, the reduction of the motion artifact in an image canbe realized by the subtraction processing. Now, a case where the motionartifact appears at a rate of, for example, 10% of a signal originatingfrom the tissues in an imaging band will be taken as an example anddescribed with reference to FIGS. 22 and 23.

FIG. 22A shows part of the spectrum of reflected waves obtained by thetransmission ultrasonic wave of the first rate in the case where theultrasonic wave has been transmitted at 2 rates by the prior-art method(that is, without performing the specified control of a differencefrequency component). Besides, FIG. 22B shows part of the spectrum ofreflected waves obtained by the transmission ultrasonic wave of thesecond rate. As seen from FIGS. 22A and 22B, an echo signal generated bya motion in the temporal difference between the rates is superposed onthe leakage component of a first fundamental wave as lies on the higherfrequency side of the fundamental wave and the lower frequency side ofthe harmonic wave thereof.

Besides, in a case where the signal of FIG. 22A is subtracted from thesignal of FIG. 22B, a noise component (motion artifact) which is causedby the motion having arisen between the first and second rates remainsas shown in FIG. 22C. In this case, the motion artifact appears about10% of the signal which originates from the tissues and which includesthe leakage component of the fundamental wave.

On the other hand, FIG. 23A shows part of the spectrum of reflectedwaves obtained by the transmission ultrasonic wave of the first rate inthe case where the ultrasonic wave has been transmitted at the 2 ratesby the method according to this embodiment. Besides, FIG. 23B shows partof the spectrum of reflected waves obtained by the transmissionultrasonic wave of the second rate. As seen from FIG. 23A, the leakagecomponent of the fundamental wave is reduced relatively to the same inthe prior art, owing to the weakening by the difference frequencycomponent, and as seen from FIG. 23B, only the echo signal generated bythe motion in the temporal difference between the rates appears on thehigher frequency side of the first fundamental wave and on the lowerfrequency side of the harmonic wave.

Besides, in a case where the signal of FIG. 23A is subtracted from thesignal of FIG. 23B, a noise component (motion artifact) which is causedby the motion having arisen between the first and second rates remainsas shown in FIG. 23C. In this case, the motion artifact appears about10% of the signal which originates from the tissues and from which theleakage component of the fundamental wave has been removed. Accordingly,the motion artifact can be made lower than with the prior-art method.

As stated above, with the method according to this embodiment, theleakage component of the fundamental wave as included in the receptionsignal is first removed or reduced by the difference frequencycomponent, and the subtraction processing between the rates isthereafter performed using the resulting reception signal so as toextract the contrast medium echo signal. Accordingly, the leakagecomponent itself of the fundamental wave and the influence of the motionartifact on the leakage component of the fundamental wave can beeliminated, and the signal originating from the tissues can be removedat a high accuracy by the subtraction processing. As a result, thecontrast medium echo component can be enhanced more relatively to thesame in the prior art, and effective blood flow information can beimaged.

While the present invention has been thus far described in conjunctionwith embodiments, one skilled in the art can suggest variousmodifications and alterations within the category of the idea of theinvention, and it is to be understood that the modifications andalterations fall within the scope of the invention. As indicated initems (1)-(3) by way of example below, the embodiments can be variouslychanged within the scope not departing from the subject matter of theinvention.

(1) In each of the embodiments, there has been exemplified the casewhere the echo signal suitable for imaging is extracted by utilizing thedifference frequency component generated on the basis of the twofundamental waves. It is also allowed, however, to adopt a configurationin which a similar operation is realized by utilizing a differencefrequency component generated on the basis of two or more fundamentalwaves.

(2) In each of the embodiments, the frequency of the second fundamentalwave whose frequency is higher in the two fundamental waves iscontrolled, thereby to control the frequency of the difference frequencycomponent which is generated on the basis of the two fundamental waves.However, this does not intend any restriction, but it is also allowed toadopt a configuration in which the first fundamental wave of lowerfrequency is controlled in accordance with the frequency band of thefundamental waves to-be-used, thereby to control the frequency of thedifference frequency component.

(3) In the first or second embodiment, it is also allowed to adopt aconfiguration in which transmissions at 2 rates with phases inverted byway of example are performed, and reception signals corresponding to therespective rates are added up, thereby to remove the fundamental wavecomponents. In accordance with such a configuration, in the firstembodiment, the second harmonic component to be imaged can be moreenlarged by the superposition of the difference frequency component andthe addition processing. Besides, in the second embodiment, the secondharmonic wave which is, in principle, double larger can be obtained bythe addition processing.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the present invention in its broaderaspects is not limited to the specific details, representative devices,and illustrated examples shown and described herein. Accordingly,various modifications may be made without departing from the spirit orscope of the general inventive concept as defined by the appended claimsand their equivalents.

1-8. (canceled)
 9. An ultrasonic diagnostic equipment comprising: atransmission ultrasonic wave generation unit which generates atransmission ultrasonic wave that has, at least, a first fundamentalwave, and a second fundamental wave at a frequency higher than that ofthe first fundamental wave, and which generates the transmissionultrasonic wave by controlling the frequency of at least one of thefirst and second fundamental waves in order that, in case oftransmitting the transmission ultrasonic wave to a patient and receivinga reflected wave therefrom, a sum frequency component between the firstfundamental wave and the second fundamental wave as is included in thereflected wave may interact with at least one of a second harmonic waveof the first fundamental wave and a second harmonic wave of the secondfundamental wave, and also by controlling a phase of at least one of thefirst and second fundamental waves in order to control the interaction;a transmission unit which transmits the transmission ultrasonic wave tothe patient; a reception unit which receives the reflected wave of thetransmission ultrasonic wave from the patient; and an image generationunit which generates an ultrasonic image on the basis of the reflectedwave.
 10. An ultrasonic diagnostic equipment as defined in claim 9,wherein said transmission ultrasonic wave generation unit generates thetransmission ultrasonic wave by controlling the phase of at least one ofthe first and second fundamental waves in order that the second harmonicwave and the sum frequency component may become inphase.
 11. Anultrasonic diagnostic equipment as defined in claim 10, wherein saidtransmission ultrasonic wave generation unit sets a phase difference ofthe second fundamental wave relative to the first fundamental wave, at πin a case where the first fundamental wave and the second fundamentalwave are of sine type; and sets a phase difference of the secondfundamental wave relative to the first fundamental wave, at 0 or πt in acase where the first fundamental wave and the second fundamental waveare of cosine type.
 12. An ultrasonic diagnostic equipment as defined inclaim 9, wherein said transmission ultrasonic wave generation unitgenerates the transmission ultrasonic wave by controlling the phase ofat least one of the first and second fundamental waves in order that thesecond harmonic wave and the difference frequency component may becomeopposite phases.
 13. An ultrasonic diagnostic equipment as defined inclaim 12, wherein said transmission ultrasonic wave generation unit setsa phase difference of the second fundamental wave relative to the firstfundamental wave, at 0 or 2π in a case where the first fundamental waveand the second fundamental wave are of sine type; and sets a phasedifference of the second fundamental wave relative to the firstfundamental wave, at π in a case where the first fundamental wave andthe second fundamental wave are of cosine type.
 14. An ultrasonicdiagnostic equipment comprising: a transmission ultrasonic wavegeneration unit which generates a transmission ultrasonic wave that has,at least, a first fundamental wave, and a second fundamental wave at afrequency higher than that of the first fundamental wave, and whichgenerates the transmission ultrasonic wave by controlling a phase of atleast the second fundamental wave in order that, in case of transmittingthe transmission ultrasonic wave to a patient and receiving a reflectedwave therefrom, a difference frequency component or a sum frequencycomponent between the first fundamental wave and the second fundamentalwave as is included in the reflected wave may cancel leakage of at leastone of the first and second fundamental waves; a transmission unit whichtransmits the transmission ultrasonic wave to the patient; a receptionunit which receives the reflected wave of the transmission ultrasonicwave from the patient; and an image generation unit which generates anultrasonic image on the basis of the reflected wave.
 15. An ultrasonicdiagnostic equipment as defined in claim 14, wherein: said transmissionunit transmits the transmission ultrasonic wave at, at least, 2 rates;said reception unit receives from the patient the reflected waves of theindividual transmission ultrasonic waves transmitted at, at least, 2rates, and performs subtraction processing between the different rates;and said image generation unit generates the ultrasonic image on thebasis of the reflected waves subjected to the subtraction processing.16. An ultrasonic diagnostic equipment as defined in claim 14, wherein:said transmission unit transmits the transmission ultrasonic wave aplurality of times for a single scanning line; said reception unitreceives a plurality of reflected waves corresponding to the individualtransmission ultrasonic waves; and said image generation unit includesan extraction unit which extracts the second harmonic wave and thedifference frequency component from each of the plurality of reflectedwaves.
 17. An ultrasonic diagnostic equipment as defined in claim 16,wherein: said image generation unit includes a Doppler processing unitwhich generates a blood flow image on the basis of the second harmonicwave and the difference frequency component extracted every reflectedwave.
 18. (canceled)
 19. An ultrasonic image generation methodcomprising: generating a transmission ultrasonic wave that has, atleast, a first fundamental wave, and a second fundamental wave at afrequency higher than that of the first fundamental wave, by controllingthe frequency of at least one of the first and second fundamental wavesin order that, in case of transmitting the transmission ultrasonic waveto a patient and receiving a reflected wave therefrom, a sum frequencycomponent between the first fundamental wave and the second fundamentalwave as is included in the reflected wave may interact with at least oneof a second harmonic wave of the first fundamental wave and a secondharmonic wave of the second fundamental wave, and also by controlling aphase of at least one of the first and second fundamental waves in orderto control the interaction; transmitting the transmission ultrasonicwave to the patient; receiving the reflected wave of the transmissionultrasonic wave from the patient; and generating an ultrasonic image onthe basis of the reflected wave.
 20. An ultrasonic image generationmethod comprising: generating a transmission ultrasonic wave that has,at least, a first fundamental wave, and a second fundamental wave at afrequency higher than that of the first fundamental wave, by controllinga phase of at least the second fundamental wave in order that, in caseof transmitting the transmission ultrasonic wave to a patient andreceiving a reflected wave therefrom, a difference frequency componentor a sum frequency component between the first fundamental wave and thesecond fundamental wave as is included in the reflected wave may cancelleakage of at least one of the first and second fundamental waves;transmitting the transmission ultrasonic wave to the patient; receivingthe reflected wave of the transmission ultrasonic wave from the patient;and generating an ultrasonic image on the basis of the reflected wave.